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(Mother’s) Milk: It Does a Body Good and Bad?

We are what we eat, but are we also what we drink? When it comes to breast-feeding infants, we very well may be. Researchers are increasingly studying the links between the early environment of a child’s life and later life outcomes for that child, with a particular focus on how mom’s biology and behavior can influence the way that children ultimately respond to stress (which has enormous implications for health across the lifespan). In a recent study, researchers tested what they refer to as “lactational programming,” which is fancy science talk for the idea that a mom can influence her child’s biological development, for better or worse, through her breastmilk. Think of it as secondhand hormones – if mom experiences stress, she’ll have higher levels of stress hormones, some of which will be passed along to her breastfeeding infant. And because infants’ bodily systems are still developing, those secondhand hormones influence the infants’ own biology and behavior.

To test this idea, the researchers collected breastmilk samples from a group of 52 mothers, all of whom had a 3-month-old infant. The research team used the breastmilk to determine how much cortisol, a stress hormone, was in the milk and potentially passed along to infants. The researchers also asked the mothers to rate their children’s temperament (e.g., "How often during the last week did the baby startle to a sudden or loud noise?”; “When frustrated with something, how often did the baby calm down within 5 min?”). More frequent startle responses and longer delays before calming down, for example, are associated with what is referred to as “negative affectivity”; such babies are often labeled as more difficult than babies that don’t startle so easily and are able to calm down more quickly after being frustrated.

Breastmilk’s cortisol levels were positively associated with infant temperament, especially negative affectivity, but only for baby girls. Baby boys’ temperaments were unrelated to breastmilk cortisol levels. In other words, more "difficult" baby girls generally had moms with higher breastmilk stress hormones. The researchers interpret this association as evidence that babies exposed to chronically higher levels of stress hormones develop more difficult temperaments. Of course, it’s also possible that more difficult baby girls stress moms out more (causing them to have higher stress hormones). Why the effects for girls versus boys? It may have something to do with the different rates at which boys’ and girls’ biological systems develop. I’d suggest it may also have something to do with the fact that parents and others interpret similar behaviors by boys and girls differently, such that one person’s negative or difficult girl might be another’s rambunctious, strong-willed, or typical boy. Regardless, there’s no doubt that having a stressed mom is no good for any baby, boy or girl. So milk may do a body good, except for when it is fortified with stress hormones.

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Grey, K. R., Davis, E. P., Sandman, C. A., & Glynn, L. M. (2013). Human milk cortisol is associated with infant temperament. Psychoneuroendocrinology, 38, 1178-1185.

Dr. Tim Loving - Science of Relationships articles | Website/CV
Dr. Loving's research addresses the mental and physical health impact of relationship transitions (e.g., falling in love, breaking up) and the role friends and family serve as we adapt to these transitions. He's a former Associate Editor of Personal Relationships and his research has been funded by the National Institute of Child Health and Human Development. 

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